The present invention relates to a method for soldering electronic components to printed circuit boards or other elements to be connected together by soldering.
The use of electronic modules, designed in the form of printed circuit boards populated with pluggable, mountable, or surface-mountable electronic components (Surface Mounted Devices, SMDs), which must then be soldered, is growing steadily. To manufacture these modules, suitably prepared printed circuit boards are populated with the corresponding components, these components being held in place possibly with an adhesive or soldering paste in preparation for the soldering process, and finally the printed circuit boards and the components are connected together permanently and conductively by a soldering process. This soldering process is currently performed, firstly, in a conventional manner with wave soldering systems, in which solder is applied to corresponding areas by one or more solder waves that contact the printed circuit boards from below, thus soldering the corresponding parts. In a second manner, components mounted with soldering paste, soldering preforms or even solder deposits require only a melting of the solder component of the paste or the solder (reflow soldering).
In both cases however, flux must be used for a good quality solder connection. Either the flux is applied prior to the actual soldering process, as in the case of wave soldering, or the soldering paste or soldering preform contains both solder and flux. Flux aids the soldering process primarily by destroying the metal oxide layer on the workpiece that adversely effects soldering, and prevents oxidation of the metals involved during the soldering process. In addition, the surface tension of the liquefied solder is reduced. Fluxes are therefore multifunctional and are accordingly comprised of a number of substances. However, after the soldering process, flux residues remain on the printed circuit boards and can lead to disadvantageous consequences. These include reduction of the insulation resistance between the traces on the printed circuit board as well as premature corrosion of the traces. For this reason, the flux residue must be removed. It is known that this can be done by treating the boards with chlorofluorocarbons or chlorocarbons to clean them after the soldering process. The use of these substances, which are Known to be environmentally hazardous (the chlorofluorocarbon ozone problem) should however be minimized or completely avoided today.
In a recent process, the necessity for fluxes is at least reduced by preceding the soldering of the boards with a pretreatment of the boards with a low-pressure plasma, this plasma being generated from a process gas using microwaves and/or glow discharge. This is shown in German Patent Document 39 36 955. After this pretreatment, however, the soldering process is still performed under normal pressure and under air or only a local, limited supply of protective gas. During the pause between pretreatment and soldering it therefore happens that since oxygen is not excluded, oxide deposition and/or oxide formation can occur at the soldered locations, which has a negative influence on the soldering results. For this reason, as a rule it is not possible in this known method to completely avoid using fluxes or additives that have the same effect.
An object of the invention is therefore to provide a soldering method that permits soldering populated printed circuit boards without using flux, with a low reject rate and high quality.
This object is achieved according to the present invention by providing that the soldering process and any additional previous, intermediate, or subsequent steps are performed under low pressure and under the plasma effect of a special process gas atmosphere. There are advantages to performing previous, intermediate, and subsequent processing steps, for example heating and cooling steps, under the same low pressure as the soldering process, namely a low pressure of less than 100 mbar, preferably 0.5 to 20 mbar.
In an advantageous embodiment, during the preheating step, pretreatment of elements to be connected by soldering is achieved by a plasma effect that is generated there or penetrates, with the same process gas being used as a rule to form the atmosphere for the preheating step as is used to form the soldering atmosphere. However, it is provided in certain other advantageous embodiments for the atmosphere during the preheating step to be an atmosphere that is different from the soldering atmosphere.
In certain especially advantageous embodiments a process gas is used that acts, at least as a plasma, in both a reducing and oxidizing manner. In this way, both oxides and other impurities, for example greasy or oily residues, can be effectively removed or kept away during the processing of the workpiece.
According to certain preferred embodiments of the present invention, especially suitable mixtures are used, comprising 0.5 to 10 vol. % oxygen, 80 to 20 vol. % hydrogen, and 20 to 80 vol. % CF.sub.4, N.sub.2 or Ar.
As is known in theory, and also according to the invention, plasma is advantageously generated from the respective process gas mixture under the prescribed low-pressure conditions by the action of microwaves, high-frequency waves, and/or a glow discharge.
In an especially advantageous embodiment of the invention, plasma is formed in the immediate vicinity of the soldering process, in other words at the soldering location, so that soldering takes place in a particularly intensive plasma atmosphere. This measure offers considerable qualitative advantages. An especially favorable implementation of this embodiment includes, as in the case of wave soldering, wiring the soldering bath with solder waves as the cathode of a glow discharge, and so plasma formation occurs directly there.
In a majority of cases, with plasma generation at the soldering location, additional independent plasma generation at another location can be eliminated, since the effectiveness of the plasma generated at the soldering location also extends to parts of the soldering system remote from the soldering location, and in this way sufficient preparation is made for soldering.
Additional advantageous embodiments of the invention are found in the additional subclaims.
A soldering system according to the invention includes a system chamber 6 that includes the soldering station and the other stations and seals them off in a gas-tight manner from the pressure atmosphere there in the plasma state.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.
With different gas atmospheres in different parts of the system, gas-flow-inhibiting structures are advantageously provided between individual stations, particularly between the preheating zone and the soldering zone.
The invention will now be described in greater detail with reference to the figures.